In practice, scanning devices for the scanning of web edges and for determination of edge position operate either according to the reflection principle or the light barrier principle. Thereby extensive adjustment procedures are necessary when one wants to switch from one principle to the other principle. In fact, in using the reflection principle, only webs can be run whose surface is capable of reflecting the light of the light source. If the web has a net or grid-like structure, then the reflection principle can be applied only when despite the structure of the web sufficient light is reflected. A light-absorbing e.g. matte black or dar web cannot be scanned according to the reflection principle, it then is necessary to operate according to the light barrier principle, under which the web interrupts the light ray of the light source, as is determined by the appropriate receiving element. The light barrier principle can however not be applied when the web has a grid or net-type structure because no definitive edge can be determined. Also, the light barrier principle does not operate satisfactorily when the edge is frayed. In this case, as long as the web is a reflecting one, the operation must use the reflection principle.